1. Field of the Invention
The invention relates to an exhaust gas purification apparatus and method for an internal combustion engine and, more particularly, to an exhaust gas purification apparatus and method for an internal combustion engine equipped with a NOx storage-reduction catalyst.
2. Description of the Related Art
There are known exhaust gas purification apparatuses for internal combustion engines in which an exhaust passageway is provided with a NOx storage-reduction catalyst that selectively traps and stores NOx from incoming exhaust gas by adsorption or absorption, or both, when the air-fuel ratio of the exhaust gas is on the fuel-lean side of stoichiometry, and that releases stored NOx and removes the NOx through reduction when the air-fuel ratio of incoming exhaust gas becomes stoichiometric or rich of stoichiometry, so that NOx present in exhaust gas during a lean air-fuel ratio operation of the engine is trapped and stored into the NOx storage-reduction catalyst and therefore emission of NOx into the atmosphere is prevented.
As the amount of storage of NOx in the NOx storage-reduction catalyst increases, the NOx trapping capability of the catalyst decreases, and the proportion of NOx that is not trapped into the catalyst but passes through the catalyst increases. Furthermore, if the storage of NOx in the NOx storage-reduction catalyst reaches a maximum amount that is storable in the catalyst, the NOx storage-reduction catalyst becomes unable to store any more NOx from exhaust gas, so that the entire amount of NOx present in exhaust gas passes through the catalyst.
Therefore, in exhaust gas purification apparatuses employing a NOx storage-reduction catalyst, a regeneration operation (rich-spike operation) of operating the engine at a rich air-fuel ratio is performed when the storage of NOx in the NOx storage-reduction catalyst increases to a certain amount. By executing the regeneration operation, a rich air-fuel ratio exhaust gas is supplied to the NOx storage-reduction catalyst, so that NOx is released from the NOx storage-reduction catalyst, and is removed through the reduction by unburned hydrocarbon, CO, etc., that are present in the rich air-fuel ratio exhaust gas. Therefore, the storage of NOx in the NOx storage-reduction catalyst reduces, and the NOx trapping capability of the NOx storage-reduction catalyst is recovered.
The regeneration operation (rich-spike operation) needs to be appropriately performed in accordance with the amount of NOx stored in the NOx storage-reduction catalyst, as indicated above. For example, if the regeneration operation is performed when the NOx storage-reduction catalyst stores only a small amount of NOx and retains a good trapping capability, increased frequency of performing the engine operation with a rich air-fuel ratio will result, so that the property of exhaust gas may degrade or the fuel consumption may increase. Conversely, if the frequency of performing the regeneration process is lower than necessary, the storage of NOx in the NOx storage-reduction catalyst will increase beyond an allowable limit, so that the NOx trapping capability of the catalyst may drop and the property of exhaust gas may degrade. Therefore, in order to appropriately perform the rich-spike operation, it is necessary to accurately find the amount of NOx stored in the NOx storage-reduction catalyst. However, in reality, it is difficult to directly measure the amount of NOx stored in the NOx storage-reduction catalyst during operation of the engine. Therefore, various methods for estimating the amount of storage of NOx in the NOx storage-reduction catalyst instead of direct measurement have been proposed.
For example, a known method for accurately estimating the amount of NOx stored in the NOx storage-reduction catalyst uses a NOx counter (Japanese Patent Application Laid-Open Publication No. 7-139340). The NOx counter is a counter value that is incremented or decremented in accordance with the state of operation of the engine all the time during the engine operation so that the value corresponds to the amount of NOx stored in the NOx storage-reduction catalyst. In an exhaust gas purification apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 7-139340, during lean air-fuel ratio operation of the engine, an amount determined in accordance with the state of operation of the engine is added to the NOx counter at constant time intervals. During operation of the engine at the stoichiometric air-fuel ratio or rich air-fuel ratio, an amount determined in accordance with, for example, the air-fuel ratio of the engine, the temperature of the NOx storage-reduction catalyst, etc., is subtracted from the NOx counter at constant time intervals. In this manner, the value of the NOx counter is changed so as to always correspond to the present amount of storage of NOx in the NOx storage-reduction catalyst.
Specifically, the amount of NOx discharged from an engine in a unit time during operation of the engine is determined in accordance with engine operation conditions, such as engine load, engine speed, etc. It is considered that during the lean air-fuel ratio operation of the engine, a certain proportion of the NOx discharged from the engine is trapped and stored into the NOx storage-reduction catalyst. Therefore, during the lean air-fuel ratio operation, the storage of NOx in the NOx storage-reduction catalyst is considered to increase, in a unit time, by an amount obtained by multiplying the amount of NOx produced from the engine by the predetermined proportion.
During the stoichiometric or rich air-fuel ratio operation of the engine, the NOx stored in the NOx storage-reduction catalyst is released from the catalyst at a predetermined rate, and is removed through reduction. The amount of NOx released from the NOx storage-reduction catalyst in a unit time, that is, the amount of decrease in the storage of NOx per unit time, is considered to be proportional to the amount of inflow of unburned fuel, CO, and the like to the NOx storage-reduction catalyst.
In the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 7-139340, the value of the NOx counter is controlled so as to accurately correspond to the present amount of storage of NOx in the NOx storage-reduction catalyst all the time during operation of the engine by increasing the NOx counter at a rate corresponding to the increase in the storage of NOx in the catalyst during the lean air-fuel ratio engine operation and by decreasing the NOx counter at a rate corresponding to the decrease in the storage of NOx in the catalyst during the stoichiometric or rich air-fuel ratio engine operation.
The apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 7-139340 accurately estimates the amount of NOx stored in the NOx storage-reduction catalyst through the use of the NOx counter value acquired as described above, thereby allowing appropriate execution of the rich-spike operation.
It should be possible to precisely execute the rich-spike operation by estimating the amount of NOx stored in the NOx storage-reduction catalyst through the use of the NOx counter as in Japanese Patent Application Laid-Open Publication No. 7-139340. However, in reality, if the rich-spike operation is executed only on the basis of the storage of NOx estimated through the use of the NOx counter as in Japanese Patent Application Laid-Open Publication No. 7-139340, there may arise a problem of the NOx storage-reduction catalyst failing to sufficiently recover the NOx trapping capability despite execution of the rich-spike operation.
The NOx storage-reduction catalyst traps and stores NOx from exhaust gas and releases NOx in accordance with the change in air-fuel ratio between the lean side and the rich side of stoichiometry. However, in reality, the rates of trap and release of NOx are not uniform over the entire body of the NOx storage-reduction catalyst, but greatly vary depending on locations in the catalyst. For example, when NOx is trapped and stored during the lean air-fuel ratio operation of the engine, most NOx is initially stored in an upstream-side portion of the NOx storage-reduction catalyst, and substantially no NOx reaches a downstream-side portion of the catalyst. Therefore, during a certain time following the beginning of the lean air-fuel ratio engine operation, NOx is unlikely to be stored in a downstream-side portion of the catalyst. Similarly, during the regeneration operation, the release of NOx from the catalyst initially occurs at the upstream side, and therefore the consumption of unburned hydrocarbons and the like contained in exhaust gas initially occurs in a region near the upstream side of the catalyst. Hence, during an early period of the regeneration operation, the release of NOx is unlikely to occur in a portion of the catalyst near the downstream side.
Thus, the upstream-side portion and the downstream-side portion of the NOx storage-reduction catalyst have greatly different rates of storage and release of NOx. In general, the storage/release of NOx initially occurs in an upstream-side portion of the catalyst, and then occurs in a downstream-side portion after the elapse of a delay time. In this manner, the characteristics (rate and timing) of storage and release of NOx are greatly different between the upstream-side portion and the downstream-side portion of the NOx storage-reduction catalyst. Similar to the differences between the upstream-side portion and the downstream-side portion of the catalyst, differences in the NOx storage and release characteristics exist between an upper coat layer portion and a lower coat layer portion of the catalyst.
The apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 7-139340 estimates the amount of storage of NOx in the NOx storage-reduction catalyst by using fixed rates of storage and release of NOx although the NOx storage/release characteristics vary depending on locations in the catalyst. Therefore, errors may occur in the estimation of the storage of NOx in the catalyst. For example, if the fixed values of the NOx storage/release rates adopted in the apparatus are close to the values of the rates of an upstream-side portion of the catalyst, the rich-spike operation for the NOx storage-reduction catalyst is ended on the basis of a determination that the release of NOx is completed, while a downstream-side portion of the catalyst has not even started to release NOx or still stores some NOx although the portion has started releasing NOx. As a result, some NOx is left in the downstream-side portion of the catalyst when the trapping and storing of NOx into the catalyst is restarted. In this manner, the storage of NOx in the downstream-side portion of the NOx storage-reduction catalyst gradually increases, resulting in a problem of a considerable drop in the trapping capability of the portion of the catalyst despite execution of the rich-spike operation based on the amount of storage of NOx estimated through the use of the NOx counter.
While the above-description is made in conjunction with NOx, SOx (oxides of sulfur) can also be trapped, if contained in exhaust gas, by the NOx storage-reduction catalyst substantially in the same fashion as NOx when the air-fuel ratio is lean of stoichiometry. The storage of SOx in the NOx storage-reduction catalyst decreases the NOx trapping capability of the catalyst as is the case with the storage of NOx. Therefore, if SOx is stored in the catalyst, it is necessary to execute a regeneration operation similar to the above-described rich-spike operation in order to recover the NOx trapping capability of the NOx storage-reduction catalyst. The generation operation for releasing SOx from the NOx storage-reduction catalyst requires that the air-fuel ratio of exhaust gas entering the catalyst be maintained on the rich side of stoichiometry as in the above-described rich-spike operation, and the exhaust gas temperature be raised higher than the temperature for the rich-spike operation for releasing NOx. Therefore, in order to fully utilize the trapping capability of the NOx storage-reduction catalyst, it is necessary to accurately estimate the amount of SOx stored in the catalyst and execute an appropriate regeneration operation for releasing SOx as in the case of NOx.